The invention disclosed broadly relates to the field of computer memory. More particularly, preferred embodiments of the present invention relate to the use of curved magnetic regions to create the needed magnetic anisotropy and coercivity for memory cells in a magnetic random access memory.
Magnetic memory cells are memory cells that store information in the orientation of the magnetization of a ferromagnetic region. These magnetic memory cells are nonvolatile and can hold stored information for long periods of time. Magnetic memory cells that use a magnetic state to alter the electrical resistance of the materials near the ferromagnetic region are collectively known as magnetoresistive (MR) memory cells. An array of magnetic memory cells is often called magnetic RAM or MRAM (magnetic random access memory). MRAM arrays include an array of magnetic memory cells positioned at the intersections of wordlines and bitlines. Each cell includes a magnetically changeable or free region and a proximate reference region arranged into a magnetic tunnel junction (xe2x80x9cMTJxe2x80x9d) device. The principle underlying storage of data in such cells is the ability to change the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis (xe2x80x9cEAxe2x80x9d) of the free region, and the ability to thereafter read this relative orientation difference. More particularly, MRAM cells are written to by reversing the free region magnetization using applied bi-directional electrical, and resultant magnetic, stimuli via its respective bitline and wordline, and are later read by measuring the resultant tunneling resistance between the bitline and wordline. The tunneling resistance assumes one of two values depending on the relative orientation of the magnetization of the free region with respect to the reference region. If the free region is modeled as a simple elemental magnet having a direction of magnetization which is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or xe2x88x92EA), and if the reference region is, for example, a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states (and therefore the two possible tunneling resistance values) are defined for the cell: aligned (+EA/+EA) and anti-aligned (xe2x88x92EA/+EA). These two states may be used to represent a logical 1 or 0 for typical binary processing applications.
Coercivity refers to the magnetic field strength required to alter the orientation of the magnetization in a magnetic memory cell. In an MRAM device, the bits should all have approximately the same coercivity so that they can all be altered using a magnetic field of approximately the same strength. In addition, it is important that a magnetic memory be designed such that when the bits are scaled to smaller dimensions they do not suffer from thermally activated switching errors due to a decrease in their volume. The traditional approach uses shape anisotropy to create this needed coercivity. Unfortunately, this approach suffers from several problems. First, it is difficult to pattern all of the junctions with exactly the same shape. This variance in the shapes of the junctions leads to a corresponding spread in the coercivities of the bits. Furthermore, large aspect ratios are required to create this shaped-induced anisotropy and the large aspect ratios of the shaped anisotropic bits decreases the density of an MRAM constructed out of the bits. Finally, as the bits are scaled down in area, the thickness of the bits must also decrease in order to maintain the required coercivity. This scaling down of the thickness of the bits results in a decrease in bit volume which increases the likelihood of thermally activated switching errors. Therefore, what is needed is an improved magnetic memory bit that has a substantially uniform coercivity and can be scaled down without introducing thermally activated switching errors.
A preferred embodiment of the present invention is directed toward a magnetic random access memory. The magnetic random access memory includes a plurality of magnetic memory cells. Each magnetic memory cell has a substrate, a reference layer, a barrier layer and a free layer having a hard axis and an easy axis. The substrate has a plurality of grooves that are substantially parallel to the easy axis of the free layer. A copper read or write wire is incorporated into at least one of the grooves such that the wire has a concave shape or a convex shape. At least a portion of the free layer is positioned on one of the grooves of the substrate to create a curved portion such that the free layer is curved with respect to the hard axis and substantially straight with respect to the easy axis. In a most preferred embodiment, the free layer comprises a shape of the arc of a circle. The reference layer is also preferably substantially curved with respect to the hard axis and substantially straight with respect to the easy axis. The barrier layer is positioned between the reference layer and the free layer.
Another embodiment of the present invention is directed toward an information processing system. The information processing system includes a processor, a memory and an input/output interface. The memory is preferably a magnetic random access memory having a substrate and a plurality of magnetic cells. Each magnetic cell has a magnetic region having an easy axis and a hard axis. The magnetic region includes a stack with a reference layer, a free layer and a barrier layer. The substrate has a plurality of grooves substantially parallel to the easy axis of the magnetic region. The magnetic region is substantially curved with respect to the hard axis and substantially straight with respect to the easy axis. Preferably, the magnetic region includes a curved free layer located over one of the grooves of the substrate. A wire for supplying read and write signals to the magnetic cells is incorporated into at least one of the grooves.
Yet another embodiment of the present invention is directed toward a method of producing a magnetic memory cell having a free layer, a barrier layer and a reference layer. In accordance with the method, the free layer is formed such that a portion of the free layer is curved. Preferably, the reference layer is also produced such that a portion of the reference layer is curved. This is accomplished by creating curved regions in a substrate layer by etching grooves in the substrate and depositing the free layer and reference layer over the curved regions to produce the curved portions. Read and write wires may also be formed in portions of the etched grooves.
The above described embodiments of the present invention substantially improve upon the prior art by providing magnetic memory cells that exhibit magnetic anisotropy, substantially uniform magnetic coercivities and are relatively easy and inexpensive to construct. In addition, the coercivity of a magnetic memory cell constructed in accordance with the above discussed embodiments is independent of the thickness of the memory cell. Thus, dense arrays of magnetic memory cells may be constructed in accordance with the preferred embodiments of the present invention without substantially decreasing their volume and, thereby, increasing the likelihood of thermally induced errors. Thus, the present invention is particularly well suited for use in applications where it is desirable to produce as small of a magnetic memory cell as possible.